Passive automatic antenna tuning based on received-signal analysis

ABSTRACT

A method includes receiving a signal from a remote transmitter via an electrically-tunable antenna having a tunable element. An adjustment, to be applied to a response of the electrically-tunable antenna, is calculating by analyzing the received signal. The response of the electrically-tunable antenna is adapted by controlling the tunable element responsively to the estimated adjustment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/873,222, filed Oct. 2, 2015, which claims the benefit of U.S.Provisional Patent Application 62/060,600, filed Oct. 7, 2014, and isalso a continuation-in-part of U.S. patent application Ser. No.14/708,255, filed May 10, 2015, which is a continuation of PCTApplication PCT/IB2015/053341, filed May 7, 2015, which claims thebenefit of U.S. Provisional Patent Application 61/991,628, filed May 12,2014. The disclosures of all these related applications are incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to wireless receivers andtransceivers, and particularly to automatic antenna tuning.

BACKGROUND OF THE INVENTION

Many wireless devices are constrained to a small volume, and thereforeuse Electrically-Small Antennas (ESAs). An ESA has physical dimensionsthat are small relative to the free-space wavelength λ. One exampledefinition regards an antenna as electrically small if it is able to fitin a sphere of radius λ/2π.

Electrically-Small Antennas are described, for example, by Wheeler, in“Fundamental Limitations of Small Antennas,” Proceedings of The IRE,volume 35, issue 12, December, 1947, pages 1479-1484; by Wheeler, in“The Radiansphere Around a Small Antenna,” Proceedings of The IRE,volume 47, issue 8, August, 1959, pages 1325-1331; and by McLean, in “ARe-Examination of the Fundamental Limits on The Radiation Q ofElectrically Small Antennas,” IEEE Transactions on Antennas andPropagation, volume 44, issue 5, May, 1996, pages 672-675, which are allincorporated herein by reference.

SUMMARY OF THE INVENTION

An embodiment of the present invention that is described herein providesa method including receiving a signal from a remote transmitter via anelectrically-tunable antenna having a tunable element. An adjustment, tobe applied to a response of the electrically-tunable antenna, iscalculating by analyzing the received signal. The response of theelectrically-tunable antenna is adapted by controlling the tunableelement responsively to the estimated adjustment.

In some embodiments, calculating the adjustment and adapting theresponse include carrying out an iterative tuning process that aims tomaximize a level of the received signal. In an embodiment, the receivedsignal includes known signal components, and calculating the adjustmentincludes deriving the adjustment from measured received levels of theknown signal components.

In a disclosed embodiment, calculating the adjustment includesestimating a slope of the response as a function of frequency over afrequency range, and deriving the adjustment from the estimated slope.In another embodiment, calculating the adjustment includes estimating achange in a slope of the response as a function of frequency between atleast first and second settings of the tunable element, and deriving theadjustment from the estimated change in the slope.

In some embodiments, calculating the adjustment includes measuringrespective levels of the received signal for at least first and secondsettings of the tunable element, and deriving the adjustment bycomparing the received levels. In an example embodiment, measuring thelevels includes performing a first measurement of the first setting, andperforming second measurements of the second setting before and afterthe first measurement, and comparing the received levels includescomparing the first measurement to a combination of the secondmeasurements.

In another embodiment, calculating the adjustment includes evaluatingtwo or more different settings of the tunable element during a singlesymbol of the received signal. In yet another embodiment, receiving thesignal includes receiving a sequence of communication time frames, andadapting the response includes controlling the tunable element atboundaries between the time frames.

In still another embodiment, receiving the signal includes operating areceiver that receives the signal in an intermittent reception mode, andthe method includes waking-up the receiver in order to calculate theadjustment in response to meeting a predefined wake-up criterion. In adisclosed embodiment, the received signal is transmitted from a firstbase station on a first frequency, and calculating the adjustmentincludes predicting a setting of the tunable element to be used whenreceiving a subsequent signal from a second base station on a secondfrequency. In an embodiment, the method includes calculating, based onthe adapted response, an antenna tuning setting to be used for signaltransmission.

There is additionally provided, in accordance with an embodiment of thepresent invention, an apparatus including an electrically-tunableantenna, a receiver and control circuitry. The electrically-tunableantenna includes a tunable element. The receiver is configured toreceive a signal from a remote transmitter via the electrically-tunableantenna. The control circuitry is configured to calculate an adjustmentto be applied to a response of the electrically-tunable antenna byanalyzing the received signal, and to adapt the response of theelectrically-tunable antenna by controlling the tunable elementresponsively to the estimated adjustment.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are block diagrams that schematically illustrate wirelesscommunication devices with adaptively-tunable antennas, in accordancewith embodiments of the present invention;

FIGS. 4A and 4B are graphs showing efficiencies of electrically-smallantennas, in accordance with an embodiment of the present invention;

FIG. 5 is a block diagram that schematically illustrates impedancetuning and aperture tuning of an electrically-small antenna, inaccordance with an embodiment of the present invention;

FIG. 6 is a diagram illustrating the use of an Orthogonal FrequencyDivision Multiplexing (OFDM) signal for antenna tuning, in accordancewith an embodiment of the present invention; and

FIG. 7 is a graph that schematically illustrates antenna tuning based onthe slope of estimated antenna efficiency as a function of frequency.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Embodiments of the present invention that are described herein provideimproved techniques for adaptive tuning of antennas in wirelesscommunication devices. Although the embodiments described herein refermainly to adaptively-tunable Electrically-Small Antennas (ESAs), whichare typically narrowband and can benefit significantly from adaptivetuning, the disclosed techniques are applicable to any other suitabletype of antenna.

In some embodiments, a wireless device comprises at least oneadaptively-tunable antenna that is used for signal reception andpossibly also for transmission. A receiver, which is possibly part of atransceiver, processes a signal that is received from a remotetransmitter via the antenna. A control unit analyzes the received signaland tunes the antenna in response to the analyzed received signal.Tuning may involve impedance tuning, aperture tuning, and/or othersuitable form of tuning, as will be described below.

The disclosed tuning schemes are entirely passive, in the sense thatthey do not use or require any signal transmission. This property is incontrast, for example, to tuning schemes that are based on VoltageStanding Wave Ratio (VSWR) measurements. In some embodiments, however,the disclosed techniques can be used in combination with active, e.g.,VSWR-based, tuning schemes.

Antenna tuning based on the received signal has numerous importantadvantages. In many cases a transmitted signal is not present at all, orat least for long periods of time. For example, some wireless devicesuse receive-only antennas that are not used for transmission at all. Inother cases, an antenna is used for transmission occasionally, but thereare long periods of time during which no signal is transmitted (e.g.,when the device is idle, during long paging cycles, during acquisitionof system information, during measurement of downlink signals, to namejust a few examples).

In yet other cases, tuning an antenna based on the transmitted signal isnot optimal, and sometimes unsuitable, for reception performance. Forexample, in some communication protocols transmission and reception areperformed on different frequencies, and therefore tuning an antenna onthe transmission frequency does not guarantee acceptable receptionperformance. Tuning the antenna based on the received signal is optimal,in the sense that is aims to maximize the reception performance on theactual reception frequencies and using the actual signal to be received.In addition, reception-based tuning typically optimizes the antennaperformance and guarantees the strongest possible received signal,whereas VSWR-based tuning typically converges to the minimal returnedsignal power and is unable to tightly control the energy dissipated inthe antenna itself.

Typically, the control unit in the wireless device carries out thedisclosed tuning schemes continuously and at a fast rate, in order totrack and compensate for time-varying effects such as body effects. Fastand continuous tuning is also important because the received signal maybe subject to fast channel fading.

In some embodiments, the tuning scheme aim to center the frequencyresponse of the antenna (e.g., the antenna resonance frequency) on theactual reception frequency. In an embodiment, the control unit estimatesthe slope of the antenna response as a function of frequency, or thechange in slope from one tuning setting to another. The estimated slopeis then used for deciding how to adapt the frequency response, e.g.,whether to increase or decrease the antenna center frequency and by whatincrement. An example method that estimates the slope by analyzingReference Signals (RSs) in different frequency bins is described indetail.

Wireless Devices with Adaptively-Tunable Electrically-Small Antennas

In some embodiments, a disclosed wireless communication device comprisesone or more Electrically-Small Antennas (ESAs) used for transmissionand/or reception. In the context of the present patent application andin the claims, the term “ESA” refers to an antenna whose volume isconfined to a sphere of radius λ/2π, wherein λ denotes the free-spacewavelength of signals transmitted or received by the antenna.

ESAs are by nature narrowband, and their bandwidth typically decreaseswith physical size. For many wireless communication applications, theinstantaneous bandwidth of an ESA (e.g., on the order of no more than 6%of the center frequency) is considerably narrower than the end-to-endbandwidth of the transmission and reception bands (sometimes on theorder of 5-25% of the center frequency).

In order to use narrowband ESAs in broadband wireless communicationapplications, the wireless device's ESAs in the disclosed embodimentsare adaptively tunable. In an embodiment, the device further comprises acontrol unit that adaptively tunes the frequency response of the ESA tothe specific narrowband frequency slice that is actually used for signaltransmission or reception. When using such an adaptively-tunable ESA,the instantaneous bandwidth of the ESA is only required to correspond tothe instantaneous bandwidth of the transmitted or received signal(usually no more than 20 MHz).

In the context of the present patent application and in the claims, theterm “adaptively-tunable antenna” refers to an antenna whose frequencyresponse can be adjusted during device operation, as opposed to tuningduring production. The antenna response can be tuned adaptively to matcha desired frequency slice or sub-band of operation, possibly undervarying conditions and circumstances. The tunable element or elements inan adaptively-tunable antenna may comprise or be coupled to the physicalradiating elements of the antenna, and/or associated circuitry. Theantenna may comprise any suitable tuning mechanisms, such as, forexample, an aperture tuning mechanism, an impedance matching network,and/or a mechanism that adaptively connects or disconnects one or moreantenna elements. The tuning scheme can be used for tuning to thedesired frequency slice, as well as for compensating for various effectsthat degrade the antenna performance or shift the antenna oscillationfrequency, such as body effects.

Various example configurations of wireless devices withadaptively-tunable ESAs are described herein. In one embodiment, thedevice comprises a transmit/receive (TX/RX) antenna and a receive-only(RX) antenna, both adaptively tunable. The RX antenna is tuned to thedesired RX sub-band, and the TX/RX antenna is tuned to the desired TXsub-band. On reception, the device performs diversity reception with theRX-only antenna serving as the primary antenna and the TX/RX antennaserving as the diversity antenna. Since the TX/RX antenna is tuned tothe TX sub-band, its gain and efficiency in the RX sub-band aredegraded. This degradation, however, is tolerable when serving as adiversity antenna.

In another embodiment, the device comprise only a singleadaptively-tunable ESA used for both transmission and reception. Whenusing Frequency-Division Duplex (FDD), for example, this TX/RX antennamay be tuned so as to balance transmission and reception performance.Alternatively, e.g., when using Time-Division Duplex (TDD) orHalf-Duplex Frequency Division Duplex (HFDD or HD-FDD), the control unitmay tune the antenna alternately to the TX and RX sub-bands as needed.

Several additional device configurations are described herein. Varioustuning schemes, and metrics that can be used by the control unit fortuning the ESAs, are also described.

The methods and devices described herein enable the use of physicallysmall antennas in broadband wireless applications, with improvedperformance for a given antenna volume, or with smaller volume for agiven performance level. The disclosed techniques can be applied in anysuitable wireless devices, and are particularly attractive in small-sizedevices such as mobile phones and wearable devices such as smart watchesand glasses.

System Description

FIG. 1 is a block diagram that schematically illustrates a wirelesscommunication device 20, in accordance with an embodiment of the presentinvention. Device 20 may comprise, for example, a mobile phone, asmartphone, a smart wearable device such as smart watch or smartglasses, a device used in an Internet-of-Things (IoT) application, orany other suitable wireless device.

Device 20 may communicate over any suitable wireless network and inaccordance with any suitable communication protocol or air interface.Example protocols comprise cellular protocols such as WidebandCode-Division Multiple Access (WCDMA), Long-Term Evolution (LTE) andLTE-Advanced (LTE-A), or Wireless Local-Area Network (WLAN) protocolssuch as the various IEEE 802.11 protocols, Alternatively, any othersuitable protocol can be used. Device 20 may operate on any suitabletransmission (TX) and reception (RX) bands, and using any suitablemultiple access scheme, e.g., Frequency-Division Duplex (FDD),Time-Division Duplex (TDD) or Half-duplex FDD (HFDD).

Although the embodiments described herein refer to wirelesscommunication devices, the disclosed techniques can also be used inother kinds of transceivers or receivers, such as navigation receiversoperating in accordance with location standards such as GPS and GLONASS.

In the present example, device 20 comprises a baseband modem 24 thatcarries out the baseband processing functions of the device, and a RadioFrequency (RF) transmitter-receiver (transceiver) 28 that performs RFtransmission and reception. Device 20 comprises two ESAs—a TX/RX antenna33 and a RX-only antenna 37. Antennas 33 and 37 may comprise anysuitable type of ESA, such as, for example, Inverted-F Antenna (IFA),Planar Inverted-F Antenna (PIFA), meander line antenna, or any othersuitable antenna type.

On transmission, baseband modem 24 generates a modulated baseband orlow-Intermediate-Frequency (IF) signal in accordance with the applicablecommunication protocol. RF transceiver 28 up-converts the signal to RF,and outputs a TX signal in a certain frequency slice in the appropriateTX band. A Power Amplifier (PA) 40 amplifies the TX signal, and aduplexer 44 filters the amplified signal. The signal then passes througha directional coupler 48 that senses the forward and reverse powerlevels. Following the coupler, the signal passes through a tunableMatching Network (MN) 34, and finally transmitted via antenna 33.

On reception, an RX signal is received from a remote transmitter (e.g.,from a base station) both by TX/RX antenna 33 and by RX antenna 37. Inthe reception chain of TX/RX antenna 33, the RX signal passes through MN34 and coupler 48. The RX signal is then filtered by duplexer 44 andprovided to RF transceiver 28. Transceiver 28 down-converts the RXsignal, e.g., to baseband or to some Intermediate Frequency (IF), andprovides the down-converted signal to modem 24 for further processingand decoding.

In the reception chain of RX antenna 37, the RX signal passes through atunable MN 38, and is then filtered by a receive filter 52. The filteredsignal is provided to RF transceiver 28, which down-converts it andprovides the down-converted signal to modem 24 for processing anddecoding.

In the embodiment of FIG. 1, baseband modem 24 comprises a control unit56 that performs various control and management functions. Among othertasks, control unit 56 tunes MNs 34 and 38, and also reads the forwardand reverse power levels using coupler 48. These tasks are used intuning the tunable TX/RX and RX-only antennas, as will be described ingreater detail below. In the present context, antenna 33 and MN 34 areregarded jointly as an adaptively-tunable TX/RX antenna 32. Similarly,antenna and MN 38 are regarded jointly as an adaptively-tunable RXantenna 36.

In a typical embodiment, signal measurements used for antenna tuning areperformed by RF transceiver 28 and provided to control unit 56. Controlfunctions, e.g., tuning the antennas based on the measurements, areperformed by control unit 56 in the baseband modem. In the context ofthe present patent application and in the claims, control unit 56 andthe circuitry in RF transceiver 24 that performs signal measurements arereferred to collectively as “control circuitry.” In this embodiment,phrases such as “control unit 56 measures a signal” mean that RFtransceiver 28 measures the signal under control of unit 56. Inalternative embodiments, the functionality of the control circuitry maybe partitioned between the baseband modem and the RF transceiver in anydesired manner, or even performed exclusively only in the basebandmodem, or only in the RF transceiver.

FIG. 2 is a block diagram that schematically illustrates a wirelesscommunication device 58, in accordance with an alternative embodiment ofthe present invention. Unlike device 20 of FIG. 1, device 58 comprisesonly a single antenna—Adaptively tunable TX/RX ESA 32.

FIG. 3 is a block diagram that schematically illustrates a wirelesscommunication device, in accordance with yet another embodiment of thepresent invention. This implementation is suitable, for example, forapplications in which transmission and reception are not performedsimultaneously, such as TDD and HFDD.

In the embodiment of FIG. 3, duplexer 44 is replaced by aTransmit-Receive (T-R) switch 57, and optional receive filter (RXF) 52and transmit filter (TXF) 58. An additional optional filter (not shownin the figure) may be inserted between RF transceiver 28 and PA 40.

The configurations of the wireless devices shown in FIGS. 1-3, and theirvarious elements, are example configurations that are chosen purely forthe sake of conceptual clarity. In alternative embodiments, any othersuitable configurations can be used. For example, the wireless devicemay comprise any other suitable number of TX/RX antennas and any othersuitable number of RX-only antennas, one or more of which antennas beingadaptively tunable. As another example, control unit 56 may beimplemented in baseband modem 24 rather than in RF transceiver 28, or inany other suitable unit of the wireless device or its host system.

The different elements of the various wireless devices of FIGS. 1-3 maybe implemented using suitable hardware, e.g., using one or moreApplication-Specific Integrated Circuits (ASICs), Field-ProgrammableGate Arrays (FPGAs) and/or RF Integrated Circuits (RFICs), usingsoftware, or using a combination of hardware and software elements.

In some embodiments, control unit 56 comprises a general-purposeprocessor, which is programmed in software to carry out the functionsdescribed herein. The software may be downloaded to the processor inelectronic form, over a network, for example, or it may, alternativelyor additionally, be provided and/or stored on non-transitory tangiblemedia, such as magnetic, optical, or electronic memory.

Use of Adaptively-Tunable Electrically-Small Antennas

As explained above, Electrically-Small Antennas (ESAs) arecharacteristically narrowband, and this property limits their usabilityand achievable performance. Consider, for example, a typical FDDapplication in which transmission and reception are performed inrespective different TX and RX bands separated by a guard band.

FIGS. 4A and 4B are graphs showing efficiencies of electrically-smallantennas, in accordance with an embodiment of the present invention.FIG. 4A shows the performance of a possible conventional solution,whereas FIG. 4B shows the performance of adaptively-tunable ESAs and 36of FIG. 1 above, in accordance with an embodiment of the presentinvention.

Both figures address an FDD application in which the RX band liesbetween 791-821 MHz, and the TX band lies between 832-862 MHz. The RXand TX bands are separated by an 11 MHz-wide guard band. Within thesebands, the wireless device receives a 10 MHz-wide RX signal in an RXslice 68, and transmits a 10 MHz-wide TX signal in a TX slice 72. The RXband comprises three possible 10 MHz-wide receive slices marked “1”, “2”and “3” in the figure, and the TX band comprises three corresponding 10MHz-wide transmit slices marked “1”, “2” and “3”. In this example, theactive channel is slice “3”, which is shaded in the figure. Thefrequencies and bandwidths above are given purely by way of example, andany other frequencies and bandwidths can be used in alternativeembodiments.

In the possible conventional solution of FIG. 4A, a dashed curve 60shows the efficiency of a TX/RX antenna, and a solid curve 64 shows theefficiency of an RX antenna. The TX/RX antenna is tuned for maximalefficiency in the guard band (see curve 60), so that the instantaneousbandwidth of the antenna would cover the RX band and the TX bandsimultaneously. The RX antenna (see curve 64) is tuned so that itsinstantaneous bandwidth would cover the entire RX band. As can be seenin the figure, this tuning scheme comes at the expense of poorefficiency of both antennas in both bands.

In FIG. 4B, a dashed curve 76 shows the efficiency of adaptively-tunableTX/RX antenna 32, and a solid curve 80 shows the efficiency ofadaptively-tunable RX antenna 36, in accordance with an embodiment ofthe present invention. Adaptively-tunable TX/RX antenna 32 is tuned formaximum efficiency over the actual 10 MHz-wide slice 72 used fortransmitting the TX signal at this particular point in time, rather thanover the entire 30 MHz-wide TX band or over both TX and RX bands.Adaptively-tunable RX antenna 36 is tuned for maximum efficiency overthe actual 10 MHz-wide slice 68 used for receiving the RX signal at thisparticular point in time, rather than over the entire 30 MHz-wide RXband.

The improvement in performance achieved by the disclosed technique canbe appreciated by comparing the corresponding efficiency curves in FIGS.4A and 4B. In this particular example, TX/RX antenna 32 has anefficiency of ˜40-45% in slice 72, as opposed to 22-25% in theconventional solution. RX antenna 36 has an efficiency of ˜40-42% inslice 68, as opposed to 34-35% in the conventional solution.

It should be noted that the efficiency of TX/RX antenna 32 drops to ˜15%in RX band 68, even though it is used for reception as well. Inpractice, however, when the device is configured to receive usingmultiple antennas, degraded reception performance can be tolerated inone of them.

For example, in some embodiments device 20 of FIG. 1 performs diversityreception, wherein RX antenna 36 serves as the primary or main antenna,and TX/RX antenna 32 serves as the diversity or secondary antenna. Insuch a configuration, the degraded efficiency of antenna 32 can betolerated, as it serves as the secondary antenna. Baseband modem 24 istypically aware of which role is played by which antenna (antenna 36serving as main, antenna 32 serving as secondary) and performs thevarious reception procedures accordingly. During device testing, when asingle RX antenna is needed, device 20 selects antenna 36 as the singleantenna.

In summary, by adaptively tuning antennas 32 and 36 to the actual TX andRX slices being used, it is possible to optimize the antennas for narrowbandwidth and thus high efficiency. In some embodiments, theinstantaneous bandwidth of the adaptively-tunable ESA should match theinstantaneous bandwidth of the signal (e.g., 20 MHz in a 20 MHz LTEsystem) rather than the bandwidth of the entire band.

In other embodiments, the adaptively-tunable ESAs may be generallynarrowband, but not necessarily as narrowband as the signal. Forexample, in an FDD or HFDD application, any antenna bandwidth that isnarrower than the applicable band (TX or RX) plus the guard band isconsidered narrowband. In a TDD application, any antenna bandwidth thatis narrower than the applicable band (TX or RX) is considerednarrowband.

In the context of the present disclosure, signal bandwidths and antennabandwidths are typically measured as 3 dB-bandwidths. Alternatively,however, any other suitable convention can be used.

Example Antenna Tuning Schemes

In various embodiments, any suitable tuning scheme or circuitry may beused for adaptively tuning ESAs 32 and 36. Some tuning schemes, referredto as impedance matching or RF matching, aim to optimize the powertransfer from the transmission line to the antenna by matching theantenna impedance to the impedance of the preceding RF circuitry.

In the examples of FIGS. 1 and 2, the antennas are tuned using RFMatching Networks (MNs) that are controlled by control unit 56. In anembodiment, control unit 56 adjusts the MNs in a closed-loop processthat aims to minimize the Voltage Standing Wave Ratio (VSWR) of theantennas. Unit 56 may estimate the VSWR, for example, by assessing theratio between the forward power level (power transmitted to the antenna)and reverse power level (power reflected from the antenna), as sensed bydirectional coupler 48.

In various embodiments, MNs 34 and 38 may be implemented using anysuitable MN topology. In some cases it is possible to select the MNtopology based on the specific type of antenna, knowledge regarding theantenna characteristics over frequency, and/or knowledge or assumptionsregarding expected body effects. For example, it may be known that theantenna impedance is expected to vary only over a known range ofimpedances (e.g., over a specific region of the Smith chart). Thisknowledge can be used in the MN design for simplifying the MN, reducelosses and enable faster convergence and smaller look-up tables incontrol unit 56. In some embodiments the MN can be simplified to asingle inductor-capacitor (LC), capacitor-capacitor (CC) orcapacitor-inductor (CL) L-shaped MN, or to a T-shaped or Pi-shaped MN.

Other tuning schemes, sometimes referred to as aperture tuning, aim tooptimize the radiation efficiency from the antenna terminals to freespace. These tuning schemes typically modify the antenna aperture and/orresonance frequency. In some embodiments, aperture tuning may beimplemented by coupling to the antenna a tunable element controlled bycontrol unit 56. A tunable element may comprise, for example, a switchedcapacitor, a tunable capacitor (e.g., barium-strontium-titanate (BST)capacitor), or a Micro Electro-Mechanical System (MEMS) device. As yetanother example, the tuning scheme may involve adaptively connecting anddisconnecting one or more antenna elements. Further additionally oralternatively, any other suitable tuning scheme can be used.

In the description above, the adaptive tuning process of control unit 56aims to optimize the antenna performance (e.g., efficiency) in thefrequency slice of interest. In addition, control unit 56 may use thetuning process to compensate for various effects that distort theantenna performance, e.g., body effects due to proximity of the userbody or other objects to the antenna.

In various embodiments, control unit 56 may tune one or more of theadaptively-tunable ESAs based on any suitable metric or combination ofmetrics. Example metrics comprise:

-   -   TX antenna VSWR measurement, as explained above    -   Properties of the RX signal such as, for example, Received        Signal Strength Indication (RSSI) for each antenna,        Reference-Signal Received Power (RSRP) for each antenna,        correlation between RX antennas, reference signal phase        difference, estimated noise/interference level in each RX        antenna and/or RX signal Modulation and Coding Scheme (MCS).    -   Properties of the TX signal, such as, for example, TX signal        power headroom.    -   Inputs from proximity detectors, indicative of nearby objects.    -   Inputs from bio sensors, e.g., heart rate, temperature, skin        moisture, blood saturation and others. In a smart-watch        application, for example, such sensors may enable control unit        56 to verify whether the wireless device is worn on the user        body or lying on a table, and also to determine the position of        the watch.    -   Inputs from motion sensors. In a smart-watch application, for        example, such inputs may be indicative of whether the device is        static, on which hand the device is worn and its orientation        (e.g., front/back of the forearm).    -   Microphone/Speaker activity sensor. If the microphone and        speaker are activated during a call, the device is likely to be        held next to the user's head.    -   Charger connection sensor.

As noted above, in some embodiments control unit 56 may tune TX/RX ESA32 while permitting a certain degradation in the ESA's receptionperformance in the RX band. In some embodiments, control unit 56 may setthe amount of permissible degradation depending on one or more of theabove metrics, or based on other suitable metrics. Control unit 56 mayalso use the above metrics, for example, to decide when tuning isneeded, and in which direction to tune.

In an alternative embodiment, control unit 56 may tune TX/RX ESA 32while balancing between TX and RX performance, based on one or more ofthe above metrics or other suitable metrics. This tuning scheme may beuseful, for example, in device 58 (having a single antenna) operating inFDD.

Non-FDD Embodiments

The description above mainly addressed FDD applications. The disclosedtechniques, however, are also applicable and advantageous in otherduplex schemes, such as Time-Division Duplex (TDD) and Half-duplex FDD(HFDD). In a TDD application, for example, transmission and receptionare performed on the same frequency, in alternating time periods. Insuch a protocol, a single TX/RX antenna can be used, such as in device58 of FIG. 2. Control unit 56 may adaptively tune TX/RX ESA 32 to theapplicable TX/RX frequency. As another example, a TDD device may use twoantennas as in device 20. In this embodiment, both antennas may benarrowband and tuned by control unit 56 using similar criteria.

In HFDD, transmission and reception are performed on differentfrequencies, but in alternating time periods and not simultaneously. Thetechniques described above with reference to FDD can be used in FDD, aswell. In some embodiments, control unit 56 may adaptively tune ESA 32depending on whether the device currently transmits or receives. Inother words, the control unit may switch the ESA to a TX-optimizedtuning scheme during transmission, and to an RX-optimized tuning schemeduring reception. Typically, the TX-optimized scheme tunes the centerfrequency of the antenna to a suitable frequency in the TX band, and theRX-optimized scheme tunes the center frequency of the antenna to asuitable frequency in the RX band.

In another embodiment, an HFDD device may use two antennas as in device20. In this embodiment, control unit 56 may alternate TX/RX ESA 32between a TX-optimized tuning scheme during transmission, and anRX-optimized tuning scheme during reception. The center frequency ofRX-only ESA 36, on the other hand, may be retained constant in the RXband.

Automatic Antenna Tuning Based on Received-Signal Analysis

In some embodiments of the present invention, control unit 56 tunes oneor more of the adaptively-tunable antennas of the wireless device basedon analysis of the signal received from the remote transmitter. Thetuning schemes described below can be used with any of the wirelessdevice configurations of FIGS. 1-3 above, or with any other suitableconfiguration.

The disclosed tuning schemes can also be used with any suitableadaptively-tunable antenna, such as, for example, the antennaconfigurations described above. Although the embodiments describedherein refer mainly to electrically-small antennas, the disclosedtechniques are not limited to such antennas and can be used for tuningof any other suitable antenna type.

FIG. 5 is a block diagram that schematically illustrates impedancetuning and aperture tuning of an antenna, in accordance with anembodiment of the present invention. The figure shows an example antennathat comprises a radiating element 98 mounted over a ground plane 94 andfed by a feedline 92.

In this example, a tunable matching network (MN) 34 matches the outputimpedance of the antenna to the input impedance of the preceding RFcircuitry. Impedance matching using MN 34 is performed based on thereceived signal. A tunable capacitor 98 is an example of a tunableelement that performs aperture tuning, i.e., tunes the resonancefrequency of the antenna. Aperture tuning using capacitor 98 (or othertunable element) is performed based on the received signal.

Any setting of MN 34 and/or capacitor 98 (or other tunable element) isreferred to herein as an antenna tuning setting. In the present exampleboth MN 34 and capacitor 98 are controlled by control unit 56, so as toapply and test various tuning settings. In alternative embodiments, thetuning scheme may apply only impedance tuning or only aperture tuning.MN 34, capacitor 98 and/or any other tuning component or circuitry arereferred to herein as a tunable element whose tuning changes theresponse of the antenna.

The following description tuning. MN 34, capacitor and/or any othertuning component or circuitry are referred to herein as a tunableelement whose tuning changes the response of the antenna.

The following description outlines several implementation examples ofantenna tuning schemes that are based on received-signal analysis. Theexamples below refer mainly to receivers operating in accordance withthe Third-Generation Partnership Project (3GPP) In Long-Term Evolution(LTE) specifications, by way of example. Alternatively, the disclosedtechniques can be used with receivers operating in accordance with anyother suitable communication or location standard or protocol. Exampleprotocols comprise Universal Mobile Telecommunications System (UMTS),Global System for Mobile communications (GSM), Code-Division MultipleAccess (CDMA), Wireless Local Area Network (WLAN) standards such as IEEE802.11 and Bluetooth, GPS, Glonass among others.

Antenna Tuning During Active Reception

In some embodiments, control unit 56 tunes the antenna during timeperiods in which the receiver receives and demodulates a desired signal.In LTE, for example, such time periods may be times during which thewireless device operates in the RRC_CONNECTED mode. Since antenna tuningchanges the antenna complex gain, it may distort the amplitude and/orphase of the received signal. Therefore, care should be taken whentuning is performed during continuous reception periods.

In an embodiment, the received signal comprises a sequence of timeframes (denoted sub-frames in LTE), and unit 56 applies tuning changesto the antenna only during boundaries between successive time frames.

In an embodiment, unit 56 decides on the appropriate tuning changes byperforming measurements on known parts of the received signal in orderto estimate the received signal level, and the slope of the receivedsignal level as a function of frequency. These two parameters areindicative of the antenna response, and the slope of the antennaresponse as a function of frequency.

In the present context, the term “known part of the signal” refers toparts of the signal whose bit values are fixed (or at least semi-fixed,e.g., remain fixed for long time duration with high probability), orotherwise known to the receiver in advance regardless of success ofdemodulation or decoding. Such known signals may comprise, for example,various types of Reference Signals (RS) and synchronization signals suchas cell-specific reference signals, Positioning Reference Signals (PRS),Physical Broadcast Channels (PBCH), Secondary Synchronization Signal(SSS), Primary Synchronization Signal (PSS), Master Information Blocks(MIB) and System Information Blocks (SIB), to name just a few examples.

In addition, it is possible to process not only serving cell signals butalso known signals from a neighbor cell operating on the same frequencyas the serving cell. This technique can boost performance, for example,under limited signal-to-noise ratio conditions. Another kind of signalthat can be used for antenna tuning is data subframes having repetition:Once the signal is successfully decoded, the other repeating subframesare known and can be used for processing.

By using the known values of such signals, unit 56 is able to obtainhigh-accuracy signal-level and slope measurements. For example, unit 56may calculate the convolution between the received known signal and theexpected known bit sequence. Such a convolution has high processinggain, allowing accurate signal-level measurement.

In the present context, the term “antenna response” refers to thetransfer function of the antenna as it is applied to the receivedsignal. The antenna response is correlative, for example, with the totalantenna efficiency and the antenna gain. All of these parameters can beexamined at a particular frequency or as a function of frequency. Theterms “antenna response,” “antenna gain” and “antenna efficiency” aresometimes used interchangeably herein. The antenna response values andtheir slope over frequency are typically used by unit 56 in determiningthe desired adjustments to the antenna tuning.

In a typical implementation, unit 56 tunes the antenna in an iterativeprocess that aims to maximize the received signal level, i.e., maximizethe estimates antenna response for the particular received signal. Ineach iteration, unit 56 estimates an adjustment to be applied to theantenna tuning, based on the measured slope of the antenna response.

In some embodiments, the adjustment comprises a frequency adjustment tobe applied to the center frequency of the antenna. This frequencyadjustment is made-up of a direction (i.e., a decision whether toincrease or decrease the center frequency) and a size (i.e., frequencyshift in the determined direction). In other embodiments, the adjustmentcomprises a direct adjustment to be applied to the tuning elements ofthe antenna (e.g., to MN 34 or to capacitor 98 of FIG. 5), withoutexplicitly estimating the resulting frequency adjustment. Furtheralternatively, any other suitable type of adjustment can be used.

Typically, the signal levels measured by unit 56 are relative. In otherwords, unit 56 may not be able to interpret the absolute signal levels,but rather to compare different settings of the antenna tuning elementsand decide which setting yields a higher received signal level.

When performing such relative measurements, care should be taken toperform the measurements under similar channel conditions. In an exampleembodiment, when measuring two tuning settings that are to be comparedwith one another, unit 56 performs one measurement on the last RS of acertain subframe, and the second measurement on the first RS of thefollowing subframe. The time proximity between the measurements reducesthe possible effects of channel fading. Alternatively, other suitablemeasurement times, which are in close time proximity to one another, canbe used.

Typically, the received signal occupies a certain bandwidth, e.g., onthe order of 10 KHz to several tens of MHz, depending on thecommunication standard. In an embodiment, unit 56 measures the receivedlevel of known signals (e.g., RS) at several frequencies across thereceived signal bandwidth. These relative measurements provide anestimate of the slope of the antenna response as a function of frequencyacross the received signal bandwidth. Unit 56 uses the estimated slopeto determine the appropriate direction and size of the adjustment to theantenna tuning setting. In an embodiment, unit 56 deduces the directionof adjustment from several slope measurements performed with differentantenna tuning settings.

The description above referred to measurements that are based on knownsignals. In alternative embodiments, unit 56 may also measure the signallevel and slope based on signal parts that carry user data or other datathat is not known in advance. For example, unit 56 may decode a datasignal, extract the error-corrected information bits from the signal,and then re-encode the data symbols. The re-encoded symbols can beregarded as a more accurate version of the received signal that sufferslittle or no distortion due to read errors. Thus, unit 56 may estimatethe antenna response from the re-encoded symbols.

Antenna Tuning During Intermittent Reception

In some embodiments, control unit 56 tunes the antenna during timeperiods in which the receiver operation is intermittent. In suchscenarios, the receiver is typically controlled to wake-up and decodesignals at a low duty-cycle, and to sleep and save power otherwise. InLTE, for example, the wireless device may operate in a DiscontinuousReception (DRX) state or in paging state, e.g., in RRC_IDLE mode. Thesleep time intervals between successive wake-up periods may be long,e.g., up to 2.56 seconds for LTE release 11 and below, and severalminutes to hours in advanced standards.

Since the antenna response may change from one wake-up period to thenext, e.g., due to body effects, it is desirable to keep tuning theantenna during the sleep intervals between wake-up periods. Whenimplementing antenna tuning during intermittent reception, care shouldbe taken to minimize power consumption, which is a prime considerationin such scenarios.

On the other hand, since the receiver does not perform actual datademodulation between wake-up periods, unit 56 is free to test tuningsettings at any suitable time during the sleep intervals. Unit 56 maychange the timing of the measurements as needed, e.g., depending onreceived signal level and/or sleep interval and wake-up perioddurations.

In an example embodiment, during intermittent reception, unit 56 changesantenna tuning settings during data symbols that do not contain knownsignals (e.g., during data symbols that do not contain RS), and performssignal-level measurements during symbols that do contain known signals(e.g., RS symbols). As noted above, since the RS are known signals,convolution between the received RS and the expected known bit sequencehas high processing gain, allowing accurate signal-level measurement.

FIG. 6 is a diagram illustrating an LTE OFDM signal received by thewireless device from a base station (eNodeB), and the way the is usedfor antenna tuning, in accordance with an embodiment of the presentinvention. The horizontal axis is a time axis in units of OFDM symbolintervals. The vertical axis is a frequency axis, in units of OFDMsubcarrier bandwidth. In the present example, each symbol isapproximately 71.3 μsec long, and the subcarriers are spaced 15 KHzapart.

Each time-frequency unit (one subcarrier over one symbol interval) isreferred to herein as a time-frequency bin. The overall received signalbandwidth may be, for example, between 180 KHz and 18 MHz. On the timeaxis, the signal is divided into subframes 100. Each subframe is dividedinto two time slots denoted “slot 0” and “slot 1”, and each slotcomprises six OFDM symbols numbered 0 . . . 6.

Some of the OFDM symbols in the received signals carry RSs in some oftheir time-frequency bins. One such symbol is marked 101 in the figure.Other symbols do not carry RSs at all. In the present example, thesignal has been transmitted by the base station from two antenna portsreferred to as TX0 and TX1. A separate pattern of RSs is transmitted viaeach antenna port—RS 102 for TX0 (marked by dark shading in the figure),and RS 104 for TX1 (marked by cross-hatching). This transmission patternis depicted purely by way of example. Any other suitable pattern can beused.

In the present example, symbols 0 and 4 in every slot are RS symbols101. In each RS symbol, RSs (of either TX0 or TX1) are received on everythird subcarrier. The remaining time-frequency bins of the receivedsignal may be used for transmitting data channels or other channeltypes, either to the wireless device or to other devices. The signalpattern of FIG. 6 is an example pattern that is depicted purely by wayof example. The disclosed techniques can be used with any other suitablesignal structure.

In some embodiments, in order to reduce the sensitivity to channelfading, control unit 56 measures the antenna response as follows. Unit56 compares two antenna tuning settings denoted A and B. A is regardedas a reference or baseline setting. Unit 56 attempts to decide whethersetting B is better or worse than setting A. This operation may beperformed, for example, in a given iteration of an iterative tuningprocess that aims to maximize the received signal level.

In the present example, unit 56 sets the antenna tuning elements tosetting A at a time 110, and performs a measurement (under setting A) onthe reference signals in the RS symbol that follows. Then, at a time112, unit 56 switches to setting B and performs a measurement (undersetting B) on the reference signals in the RS symbol that follows. At atime 114, unit 56 switches back to setting A and again performs ameasurement (under setting A) on the reference signals in the RS symbolthat follows. As explained above, changing the tuning setting isperformed during data (i.e., non-RS) symbols, whereas signal-levelmeasurements are performed during RS symbols.

In the example above, unit 56 measures setting A before and aftermeasuring setting B, and considers both measurements of setting A in thecomparison with setting B. This mechanism helps to minimize distortioncaused by channel fading. In an embodiment, unit 56 interpolates theresults of the two measurements of setting A, and compares theinterpolation result with the measurement of setting B. Thus, unit 56interpolates the first and third RS symbols (symbol 0 of slot 0 andsymbol 0 of slot 1). This interpolation approximates the result thatsetting A would have yielded on symbol 4 of slot 0 (on which setting Bis measured).

In alternative embodiments, unit 56 may interpolate a larger number ofmeasurements of RS symbols, and/or measure a larger number of possiblesettings, in order to improve the measurement accuracy. In someembodiments, e.g., in slow fading scenarios, it may be sufficient tocompare symbol 0 of slot 0 (under setting A) and symbol 4 of slot 0(under setting B), i.e., refrain from measuring setting A twice.

In addition to measuring signal level, unit 56 may also measure theslope of the antenna response as a function of frequency for a givenantenna tuning setting. The slope may be indicative of how well theantenna is currently tuned, and/or in which direction and/or by whatincrement the antenna center frequency should be adjusted.

Unit 56 typically measures the slope by estimating the relative measuredamplitudes of the different RSs in the RS symbol. In the signal of FIG.6, for example, a given RS symbol contains RSs (102 or 104) on everythird subcarrier across the signal bandwidth. Measuring the relativeamplitudes of the RSs in the symbol provides the slope (or othercharacteristic) of the antenna response as a function of frequency.

In addition to the slope in a given setting, unit 56 may also estimatethe change in slope between tuning settings. The change in slope betweendifferent tuning settings provides information for a wider bandwidth,and may provide a more accurate indication of the amplitude anddirection of the desired adjustment.

FIG. 7 is a graph that schematically illustrates antenna tuning based onthe slope of estimated antenna efficiency as a function of frequency. Agraph 120 plots the overall antenna efficiency as a function offrequency for a given antenna tuning setting. The current setting, forexample, is well tuned to receive a signal around a center frequency of˜772.5 MHz.

In most practical cases, the slope across a given channel bandwidth maybe roughly classified as flat, mild positive, mild negative, steeppositive, steep negative or concave. A concave slope typically indicatesthat the antenna is tuned or nearly tuned, and that only minoradjustments may be required. A steep positive slope typically indicatesthat the antenna center frequency should be reduced. A steep negativeslope typically indicates that the antenna center frequency should beincreased. A mild slope may either indicate that the antenna is tuned,or that the tuning is completely off-mark so that large tuning steps areneeded.

During intermittent reception (e.g., DRX or paging cycles), unit 56 mayuse various criteria to decide when it is time to wake-up the receiverin order to evaluate (and possibly adjust) the antenna tuning setting.The wake-up criterion may consider, for example, the value of the mostrecent Reference Signal Received Power (RSRP) or Reference SignalReceived Quality (RSRQ). If the received signal was strong, the antennatuning may be tracked with reduced accuracy and less frequently.

As another example, the wake-up criterion may consider low-currentsensors of the wireless device, such as proximity sensor, accelerationsensor, gyroscope and/or touchscreen. In an embodiment, unit 56initiates tuning of the antenna in response to environmental changessensed by these sensors. Such changes may indicate, for example, changesin body effects that warrant re-tuning of the antenna.

As yet another example, the wake-up criterion may consider the durationof inactivity of the wireless device. A long duration of inactivitytypically indicates a higher need for verifying the antenna tuning priorto actual reception, and vice versa. Further alternatively, unit 56 mayconsider any other suitable parameter in deciding when to activate thereceiver and evaluate the antenna tuning setting.

The description above refers to two tuning scheme, one used duringcontinuous reception and the other used during intermittent reception.In some embodiments, control unit 56 is aware of the current operationalmode of the wireless device and of transitions between modes. Unit 56may select one of the two tuning schemes using this information.

Additional Embodiments and Variations

In the example of FIG. 6 above, unit 56 switches between differenttuning settings between RS symbols, and measures the antenna responseduring RS symbols. In such an implementation, a given RS symbol istypically used in its entirety for evaluating a given tuning setting.If, however, the processing gain provided by the RS is large enough, itis possible to switch between settings during a given RS symbol, andthus use a given RS symbol for evaluating more than a single tuningsetting. Such a scheme is faster and more resilient under fast fadingscenarios.

In LTE, for example, the RS sequence is padded with zeros and isperiodic in the time domain. Therefore, it is possible to split the RSsymbol (in time) into several measurements and in this way speed-up themeasurement process. The periodicity in the time-domain of the LTE RS isone third of the OFDM symbol time. Thus, for example, unit 56 maymeasure setting A during the first third of the RS symbol, switch tosetting B during the second third of the RS symbol, and measure settingB during the third of the RS symbol. Unit 56 may then correlate theresults of the two measurements, and average the correlation result soas to produce the desired direction of tuning.

In some embodiments, unit 56 attempts to find a suitable tuning schemeduring initial acquisition of the base station signal by the receiver.In an embodiment, during initial acquisition, unit 56 may attempt toscan over several settings of the tuning elements that sufficientlycover the potential variation of the antenna resonance frequency. Theinitial acquisition process may start at these nominal settings. If nosignal is found, other settings can also be searched, e.g., startingwith the higher-probability setting. Once a base station signal is found(e.g., a PSS signal), finer tuning can be performed using the iterativeprocess described above. Initial tuning can also be performed on anysignal found during the initial search.

At a given point in time, the wireless device is served by a certainbase station on a certain frequency, and thus has good knowledge of theoptimal antenna tuning setting for this frequency. At some point thedevice may search for an alternative base station on a differentfrequency. In some embodiments, control unit 56 may use the known tuningsetting of the current serving base station to estimate the tuningsetting for the frequency of the new base station (due to expectedcorrelation between the frequency shifts).

The above mechanism is mostly applicable to intra-band cell search,i.e., when the current base station and the new base station operate inthe same frequency band.

For inter-frequency inter-band search (i.e., when the current basestation and the new base station operate in different frequency bands),depending on frequency difference and characteristics of the antenna,either the above-described initial acquisition process or theabove-described intra-band estimation process can be used.

In some embodiments, control unit 56 applies both the disclosed(reception-based) tuning schemes, as well as transmission-based tuningschemes that tune the antenna based on the transmitted signal. Thereception-based tuning yields a tuning setting referred to as “RXsetting,” and the transmission-based tuning yields a tuning settingreferred to as “TX setting.” At a given point in time, control unit 56may choose between the RX and TX settings, or combine the two settings,e.g., depending on the communication protocol being used.

For example, in FDD operation, for an RX-only antenna unit 56 uses theRX setting estimated for that antenna. For a TX/RX antenna, unit 56 useseither the TX setting, or the RX setting, or a setting that compromisesbetween the two, to match the applicable user scenario. For example, forreception-intensive scenarios (e.g., file download or video streaming),higher weight may be given to the RX scheme, and vice versa. Thecompromise may also depend on channel conditions. For example, whenreception is interference-limited, the RX setting may be of littlebenefit. The TX setting, on the other hand, may be more power efficient.

In HFDD operation, during reception unit 56 uses the RX setting. Duringtransmission unit 56 uses the TX setting if one exists, or estimates anapproximate TX setting based on the RX setting. In TDD operation, sinceboth transmission and reception are performed on the same frequency,unit 56 may use the RX setting at all times, and transmission-basedtuning may be omitted altogether.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and sub-combinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art. Documents incorporated by reference in the present patentapplication are to be considered an integral part of the applicationexcept that to the extent any terms are defined in these incorporateddocuments in a manner that conflicts with the definitions madeexplicitly or implicitly in the present specification, only thedefinitions in the present specification should be considered.

1. A method, comprising: receiving a signal from a remote transmittervia an electrically-tunable antenna having a tunable element;calculating an adjustment to be applied to a frequency-response of theelectrically-tunable antenna, by analyzing the received signal; andtuning the frequency-response of the electrically-tunable antenna bycontrolling the tunable element responsively to the estimatedadjustment.
 2. The method according to claim 1, wherein calculating theadjustment and tuning the frequency-response comprise carrying out aniterative tuning process that aims to maximize a level of the receivedsignal.
 3. The method according to claim 1, wherein the received signalcomprises known signal components, and wherein calculating theadjustment comprises deriving the adjustment from measured receivedlevels of the known signal components.
 4. The method according to claim1, wherein calculating the adjustment comprises estimating a slope ofthe frequency-response as a function of frequency over a frequencyrange, and deriving the adjustment from the estimated slope.
 5. Themethod according to claim 1, wherein calculating the adjustmentcomprises estimating a change in a slope of the frequency-response as afunction of frequency between at least first and second settings of thetunable element, and deriving the adjustment from the estimated changein the slope.
 6. The method according to claim 1, wherein calculatingthe adjustment comprises measuring respective levels of the receivedsignal for at least first and second settings of the tunable element,and deriving the adjustment by comparing the received levels.
 7. Themethod according to claim 1, wherein receiving the signal comprisesoperating a receiver that receives the signal in an intermittentreception mode, and comprising waking-up the receiver in order tocalculate the adjustment in response to meeting a predefined wake-upcriterion.
 8. The method according to claim 1, and comprisingcalculating, based on the tuned frequency-response, an antenna tuningsetting to be used for signal transmission.
 9. Apparatus, comprising: anelectrically-tunable antenna comprising a tunable element; a receiverconfigured to receive a signal from a remote transmitter via theelectrically-tunable antenna; and control circuitry, which is configuredto calculate an adjustment to be applied to a frequency-response of theelectrically-tunable antenna by analyzing the received signal, and totune the frequency-response of the electrically-tunable antenna bycontrolling the tunable element responsively to the estimatedadjustment.
 10. The apparatus according to claim 9, wherein the controlcircuitry is configured to calculate the adjustment and tune thefrequency-response in an iterative tuning process that aims to maximizea level of the received signal.
 11. The apparatus according to claim 9,wherein the received signal comprises known signal components, andwherein the control circuitry is configured to derive the adjustmentfrom measured received levels of the known signal components.
 12. Theapparatus according to claim 9, wherein the control circuitry isconfigured to estimate a slope of the frequency-response as a functionof frequency over a frequency range, and to derive the adjustment fromthe estimated slope.
 13. The apparatus according to claim 9, wherein thecontrol circuitry is configured to estimate a change in a slope of thefrequency-response as a function of frequency between at least first andsecond settings of the tunable element, and to derive the adjustmentfrom the estimated change in the slope.
 14. The apparatus according toclaim 9, wherein the control circuitry is configured to measurerespective levels of the received signal for at least first and secondsettings of the tunable element, and to derive the adjustment bycomparing the received levels.
 15. The apparatus according to claim 9,wherein, when the receiver operates in an intermittent reception mode,the control circuitry is configured to wake-up the receiver in order tocalculate the adjustment in response to meeting a predefined wake-upcriterion.
 16. The apparatus according to claim 9, wherein the controlcircuitry is configured to calculate, based on the tunedfrequency-response, an antenna tuning setting to be used for signaltransmission.